This invention relates generally to systems capable of initiating the polymerization of (meth)acrylic monomers. Specifically, this invention relates to the addition of a work-life extending agent to polymerizable (meth)acrylic monomers, as well as polymerizable systems made therewith.
Systems for initiating the polymerization of (meth)acrylic monomers to make adhesive compositions are known in the art. U.S. Pat. Nos. 5,106,928, 5,286,821, and 5,310,835, for example, disclose a two part initiator system that is reportedly useful in (meth)acrylic adhesive compositions. The first part of this two part system includes a stable organoborane amine complex and the second part includes an activator. The activator liberates the organoborane compound by removing the amine group, thereby allowing the organoborane compound to initiate the polymerization process.
The organoborane compound of the complex has the general formula: 
where R1, R2 and R3 are either alkyl groups having 1 to 10 carbon atoms or phenyl groups. Useful amine compounds for the complex include n-octylamine, 1,6-diaminohexane, diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine, ammonia, 1,3-propylenediamine, and 1,2-propylenediamine.
U.S. Pat. No. 5,286,821 reports that suitable activators for liberating the organoborane compound include aldehydes having the general structure:
Rxe2x80x94(CHO)x
wherein R is alkyl of 1 to 10 carbon atoms or aryl having 6 to 10 carbon atoms and x is 1 to 2. Examples include benzaldehyde, o-, m-, p-nitrobenzaldehyde, 2,4-dichlorobenzaldehyde, p-tolylaldehyde, and 3-methoxy-4 hydroxybenzaldehyde.
U.S. Pat. No. 5,310,835 reports that suitable activators also include organic acids having the structure:
Rxe2x80x94COOH
wherein R is H, alkyl, or alkenyl having 1 to 8 carbon atoms. Examples include acrylic acid, methacrylic acid, benzoic acid, and p-methoxybenzoic acid.
Fujisawa, Imai, and Mashuhara also describe a system for initiating the polymerization of methyl methacrylate. See Reports of the Institute for Medical and Dental Engineering, 3, 64 (1969). The system comprises a trialkylborane amine complex and an activator such as the chloride of methacrylic or n-butane sulfonic acid, terephthalic acid chloride, benzoyl chloride, p-toluene sulfonic acid chloride, benzene sulfonic acid chloride, methane sulfonic acid chloride, toluene diisocyanate, adipic acid chloride, o-tolyl isocyanate, acetyl chloride, and anhydrous acetic acid. The initiator system is reportedly useful in providing fast curing resins for dental applications.
With increasingly demanding end-user requirements, adhesive composition formulators are constantly being challenged to improve both application performance (e.g. work-life, rate of strength increase and cure time) and physical property performance (e.g. T-peel strength) of adhesive compositions. It is often times the case that a formulation change enhances one property of an adhesive composition while diminishing a second property of the adhesive composition.
In many industrial and consumer applications for adhesive compositions a long work-life is highly desirable. Work-life refers to the maximum time period available for bringing the adhesive composition into contact with the substrate(s) to be bonded (i.e. closing the bond) after the initiation of the cure of the adhesive composition. If the substrates are brought into contact with the adhesive composition after the work-life has expired, the ultimate strength of the bond formed between the substrates may be compromised.
Several techniques have been reported for increasing the work-life of polymerizable systems. In one known technique, slowing the cure rate of the polymerizable system, for example by reducing the amount of polymerization initiator in the polymerizable system and/or the chemical reactivity of the initiator, increases work-life. This technique, however, may typically lengthen the overall cure time, but may slow the rate of strength increase of the polymerizable system.
The addition of certain polymerizable monomers to polymerizable systems has also been reported to increase work-life. U.S. Pat. No. 5,859,160 (Righettini et al.) reports a free radical curable composition, useful as a two part adhesive, that includes a free radical curable compound and a vinyl aromatic compound that is chemically different than the free radical curable compound. The vinyl aromatic compound is present in an amount that is reportedly sufficient to decelerate the cure rate of the free radical composition without adversely effecting completion of cure and the properties of the curable composition after it has cured.
A need therefore exists for an adhesive having an increased work-life while maintaining other properties, such as rate of strength increase, cure time and T-peel strength.
The present invention is directed to a polymerizable system comprising an organoborane, at least one polymerizable monomer, and a work-life extending agent. The work-life extending agent may be according to one of the general formula: 
wherein R40 is CH2xe2x95x90 or alkenyl, Z1 and Z2 are independently O, Nxe2x80x94R41 or S, and R41, R42 and R43 are independently H, alkyl, aryl or cycloalkyl, provided that when Z1 and Z2 are O, R42 and R43 are independently alkyl, aryl or cycloalkyl.
The work-life extending agent may also be according to the general formula: 
wherein R44 is CH2xe2x95x90 or alkenyl and X is S or Nxe2x80x94R50, where R50 is hydrogen, alkyl, aryl or cycloalkyl.
Additionally, the polymerizable system of the invention may also include a complexing agent comprising a material selected from amines, amidines, hydroxides, alkoxides, and combinations thereof. In such a case, the work-life extending agent may be according to the general formula: 
wherein R44 CH2xe2x95x90 or alkenyl and X is O and the ratio of anhydride groups in the work-life extending agent to amine, amidine, hydroxide or alkoxide groups in the complexing agent(s) is over 3.0:1.0.
The work-life extending agent may also be of the general formula: 
wherein R40 is CH2xe2x95x90 or alkenyl, R42 is H or alkyl, R43 is H, and Z1 and Z2 are O and the ratio of amine-, amidine-, hydroxide- or alkoxide-reactive groups in the work-life extending agent to amine, amidine, hydroxide or alkoxide groups in the complexing agent(s) is over 3.0:1.0.